1. Field of the Invention
The invention relates to electrostatographic document production machines, and more particularly to automatic adjustment of parameters influencing the output reproduction of such machines.
2. Description of the Prior Art
In electrostatographic document production machines such as printers and copiers, image contrast, density, and color balance (in color machines) can be adjusted by changing certain process control parameters such as primary voltage V.sub.O, exposure E.sub.O, development station electrode bias voltage V.sub.b, the concentration of toner in the developer mixture, and the image transfer potential.
Techniques exist for regulating electrostatographic machine process control parameters so as to compensate for long term variations in the electrostatographic process. The phrase "long term" pertains to variations which would affect many successive images, and includes variations caused by such things as changes in toner concentration, wear of the image transfer member, aging of the exposure lamp, and atmospheric conditions.
Prior art systems attempt to diminish the adverse effects of these variations using manual or automatic set-up systems, or a combination of both. In typical manual set-up systems, a skilled operator examines the output reproduction (copy or print) and the corresponding input and output density (D.sub.in -D.sub.out) curves for red, green, blue, and black. Based on experience with the equipment, the operator determines adjustments to the process control parameters. Several iterations of adjustment may be required to achieve acceptable color reproductions, in terms of color balance, color fidelity, and tone reproduction.
U.S. Pat. No. 4,894,685 which issued to Shoji on Jan. 16, 1990 discloses a method for setting color process control parameters by forming a plurality of different density patches in a non-transfer portion of a photoconductor. Process control parameters are set in accordance with the differences between the recorded densities and "ideal case" densities referred to herein as "aim" densities. U.S. Pat. No. 4,647,184 which issued to Russell et al on Mar. 3, 1987 relates to a set-up mode wherein test patches of varying density are compared to stored values. The operating process control parameters are adjusted in an iterative process until there is convergence along three separate points of a stored aim curve of the photoconductor's response to exposure.
The condition of the image receiver will affect the density readings. For example, the toned patch density readings may be affected by the amount of wear of the image receiver and the existence of scumming. In the past, attempts have been made to reduce potential errors by the provision of an untoned patch in the general vicinity of the test patch. Density readings from the untoned patch are subtracted from those of the toned patch to compensate for any portion of the test reading not due to the value sought to be determined.
This process has been only partially successful due to non-uniformities in the amount of wear and scumming from one part of the image receiver to another, and further due to variations in the various coatings applied to the image receiver during manufacture. This is especially a problem when the density readings are obtained by transmission densitometers, wherein the density is determined by the amount of light transmitted through a web-type image receiver. See for example commonly-assigned U.S Pat. No. 4,693,592 which issued to V. Kurpan on Sept. 15, 1987.